1. Field of the Invention
The present invention relates to an X-ray CT (computerized tomography) apparatus and a control method of an X-ray CT apparatus which can set an appropriate tube current to a different portion of a helical scan performed once by preliminarily designating an index value to show a dispersion of CT values in required area in a reconstructed image.
2. Description of the Related Art
An X-ray CT apparatus provides a sectional image of a portion to be diagnosed of a patient by performing a scan by irradiating an X-ray to the portion to be diagnosed from a plurality of directions and performing an image reconstructing process based on projected data obtained by the X-rays from respective directions passing through the portion to be diagnosed. It is necessary for an operator such as a technician and the like to make a plan in conformity with a portion to be diagnosed and a diagnosis purpose as to scan conditions and condition of the image reconstructing process prior to the scan and the like. This plan is called a scan plan. The X-ray CT apparatus generally provides a user interface environment for a scan plan performed on an operation console.
Incidentally, since a portion to be diagnosed, for example, a lung occupies almost all the portions of a breast portion, an amount of X-ray attenuation is small, whereas since many organs exist in an abdominal portion, an amount of X-ray attenuation is large. Further, since patients have a different body shape, even the same portion has a different amount of X-ray attenuation depending on the patients.
There is known a so-called “Auto mA” technology which automatically controls a tube current value applied to an X-ray tube for determining an amount of X-ray for each patient or each scan position to cope with an amount of X-ray attenuation which is different in each patient or each portion to be diagnosed. If this technology is used, since an appropriate amount of X-ray can be set in accordance with a portion to be diagnosed having a different amount of X-ray attenuation, a tomographic view having good S/N can be obtained while suppressing wasteful exposure. Further, this technology has an advantage in that it contributes to prolong the life of the X-ray tube. Whether an “Auto mA” function is turned ON or OFF can be designated when the scan plan is made. When the “Auto mA” function is turned ON, the tube current value to be applied to the X-ray tube is automatically calculated for each set scan position. Since a calculation result is shown in a format in which a scan position and a tube current value corresponding to the scan position are described as a list, the calculation result can be confirmed by the list.
However, if the calculation result is only shown by numerical values as the list, there is a problem in that it is difficult to grasp the relation between a set scan position and a tube current value. In particular, a recent “Auto mA” function has a plurality of modes such as a mode requiring high image quality, a mode requiring a smallest exposure amount, and an operator can select these modes. In this case, if the calculation result is only shown by numerical values as the list, a problem arises in that it is difficult to confirm, if the operator changes a mode, how the calculation result is changed.
Further, the tube current values calculated by “Auto mA” may include values which are not necessarily appropriate if they are individually examined. Conventionally, in these cases, there is no unit capable of individually correcting the calculated tube current values to arbitrary values.
To cope with the above problem, a positioning image obtained by a scout scan is shown as an X-ray CT function, and the image of a graph showing the correspondence relation to the tube current values as a control value of the amount of the X-ray of the X-ray tube at respective scan positions calculated by the “Auto mA” function is shown simultaneously with the positioning image being shown in a scan plan screen for setting the scan position of a subject to be diagnosed. Further, there is a technology for making it possible to change the tube current value at an arbitrary point on an image of a graph by dragging the point using a mouse (refer to, for example, Japanese Patent Application Publication No. 2002-177261).
However, the conventional technology assumes only to display a positioning image of a single imaging surface as a superimposed display. The conventional technology is not disadvantageous if the same current value is output while a CT scan is performed in one rotation. However, if a CT scan is performed in one rotation while changing the current value, the conventional technology can not show the change of the current value.
Further, an operator designates a standard deviation as an input value for determining “Auto mA”. However, since the conventional technology displays only the image of the graph of a tube current value, if the tube current is changed, it cannot be anticipated that to what amount the standard deviation value is set. Further, it is difficult to sensuously grasp the relative relation between the variation ratio of the tube current value and the variation ratio of the standard deviation (how much change of mA results in how much change of standard deviation).